Re: [Qemu-devel] TCG register allocator

[Xin Tong]

as I mentioned. In my current implementation of coremu, the code could
be executed when it is modified. so the modifications need to be
atomic. I think I need a scratch area in which the restore_cpu_state
needs to be generated while leaving the already generated code in
tact. would this solve the problem ?

Thanks

Xin


On Wed, Jan 25, 2012 at 2:23 PM, Peter Maydell <address@hidden> wrote:
> On 25 January 2012 19:10, Xin Tong <address@hidden> wrote:
> Peter Maydell wrote:
>>> cpu_restore_state() calls gen_intermediate_code_pc() to
>>> request a retranslation of the TB with extra info to allow
>>> us to do a host-PC-to-guest-PC lookup
>>>  * Note that gen_intermediate_code_pc() overwrites the generated
>>> code that already exists in memory, and stops as soon as it
>>> reaches the point of the exception. This is harmless because
>>> we are just rewriting the same bytes to memory that were there
>>> already, but disastrous if...
>
>> in the case you described above, which code guarantees that the
>> re-generated code is laid down in the place where the old code is.
>> This has to be do somewhere in tcg.
>
> Actually, it's not in TCG but in the bit of QEMU which calls TCG.
>
> When we first translate a block of code we decide where its
> generated code will sit (in exec.c:tb_gen_code()) and set tb->tc_ptr
> to point at this location. Then when translate-all.c:cpu_gen_code()
> does this first translation it passes tcg_gen_code() a gen_code_buf
> argument which is this tb->tc_ptr.
> Later if we need to do a restore, translate-all.c:cpu_restore_state()
> calls tcg_gen_code_search_pc(), and again it passes a gen_code_buf
> argument for where generated code should be written. It uses tb->tc_ptr
> for the TranslationBlock, so it will always be at the same place.
>
> -- PMM